Rheostat



Sept. 30, 1924. 1,509,976

N. E. NORTH I RHEOSTAT 2 Sheets-Sheet l INVENTOR WlTNESES://

? Max/m5 A urf/z 7 A'TTORNEY Sept. 30, 1924. 1,509,976

v N. E. NORTH RHEOSTAT Filed April l, 1922- 2 Sheets-Sheet. 2

WITNESSES: INVENTOR War/27405 War/77. a 1 BY ATTORNEY Patented Sept. 30, 1924.

UNITED STATES" 4 1,509,976 PATENT OFFICE.

I NORMAN E. NORTH, OF CAN'ILEY, ENGLAND, ASSIGNO'R TO WESTINGHOUSE ELEC- ERIC & MANUFACTURING COMPANY,

A CORPORATION OF PENNSYLVANIA.

BHEOSTAT.

Application filed April 1,,

To all whom it may concern:

Be it known that I, NORMAN E. NORTH, a subject of the King of Great Britain, and a resident of Cantley, in the county of Ches- 5 ter, England, have invented a new and useful Improvement in Rheostats, of which the following is a specification. p

This invention relates to rheostats and motor control systems in which they are employed, and, particularly, to rheostats which are used in connection with electric motors for starting purposes such as, for instance, in electric traction.

In rheostats as ordinarily constructed having a plurality of resistances which are progressively cut out of and into circuit, the various resistances are not all in'use for the same length of time and are generally not all traversed by the same amount of current.

For example, the resistance which is the first to be cut out will be used less than that which is cut out last and furthermore when the whole of the resistances are in circuit 7 the current through the rheostat is less than it is when some only of the resistances are included in the circuit. It will be clear,

therefore, that the resistance material is 'not all used equally and some portions may not be used to their maximum extent, that is not so economically as other portions. Furthermore, cumbrous and costly contactor switches are frequently necessitated for changing the resistance value.

The object of the present invention is to provide a regulating rheostat in which the resistance material is always all in use and is also used with the same currents for substantially the same period of time and to eliminate as far as possible the switches above referred to.

According to the invention, the improved rheostat comprises a plurality of resistances all of which are approximately of the same resistance and current-carrying capacity. Said resistances are connected in a closed circuit and the junction points'of adjacent resistances are connected to contact blocks. The resistance is included in circuit by means of two terminal contacts, hereinafter for convenience termed brushes, adapted to make connection with any of the blocks at will. It will be readily seen that when the two brushes make connection with the contact blocks in such a manner that the two parallel branches of the rheostat are of equal 1922. Serial No. 548,748.

resistance, the maximum amount of resistance will be inserted. If one or other or both of'the brushes be moved so that the resistance in one branch becomes less than the resistance in the other branch, then the total resistance will be reduced but obviously the current through the branch of small resistance will be greater than the cur rent through the branch having the larger resistance. In order, therefore, that all portions of the resistance shall be equally utilized, the terminal brushes at the desired distance apart are caused to move relatively to the contact blocks of the resistance so that each portion thereof is successively moved from one parallel branch to the other.

To enable the invention to be clearly understood, it will now be described with reference to the accompanying drawings, in

which Figure 1 is a diagram of one form of the rheostat according to the invention and Fig. 2 is a diagram of modified form;

Fig. 3 is a plan view of one form of starting rheostat and associated mechanism constructed in accordance with my invention;

Fig. 4 is a view in perspective of another constructional form of rheostat and associated mechanism; and

Figs. 5 and 6 are electrical diagrams of motor control systems in which a rheostat constructed in accordance with my invention is employed.

Referring to Fig. 1, the resistance 1 in the form of grids, plates or wire or other suitable resistance elements, is connected in a closed circuit or ring and a plurality of taps 2 connect equal elements or steps of the resistance to a corresponding number of associated contact blocks 3 in contact with which the terminal blocks 4 and 5 are disposed. The terminal' blocks 4 and 5 and the associated contact blocks 3 move continuously relatively to one another, the said contact blocks being advantageously arranged in a circle to constitute a commutator of the radial or drum type. Normally, the brushes 4 and 5 occupy positions diametrically opposite one another, for example, along the dotted lines 6, 7, wherein the resistance circuit comprises two equal parallel branches and wherein the resistance value is a maximum. One of the brushes, for example, the brush 5, is adapted to be progressively moved relatively to the other brush 4.

In a position of the brush 5, such as indicated by the dotted line 8, the current pass ing through the brushes traverses resistance paths of olifi'erent lengths, namely, the

I of the resistance to be equally efiectively employed since every element thereof will have been treated in an equal way as regards the current passing through them in a given time.

lit will thus be apprecia that after one complete revolution the direction of relative rotation between the brushes and commutator, may be continued in the same direction or reversed. This being the case, instead of the circular arrangement of contact blocks, as shown in Fig. 1, a contact face plate may be used as shown in Fig.

2, the said face plate being straight as shown, or arcuate, and a relative reciprocation or oscillation being efl'ected between the plate and the pair of brushes 4 and 5, which latter are also capable of being moved to and awe. from one another, as already stated. With this arrangement in order to maintain the conditions hereinbefore set forth, the lace plate has 50% to 100% more contacts than the nber of re sistance elements or steps, so that, by connecting each resistance step to two cont blocks by means of a looping circuit 11, the equivalent of the closed loop is again provided. The operation of this form or the rheostat is similar to that shown in Fig. 1, except that the contact blocks 3 and the brushes 4 and 5 continuously reciprocate relative to one another. 4

Referring now to Fig. 3, which shows more or less diagrammatically one constructional arrangement of the rheostat, wherein the oommutator 3 is fined upon a suitable support (not shown) whilst thebrushes t and 5 are mounted upon brackets 12' and 13, respectively, which are arranged to be rotated around the commutator 3.. The resistance (not shown) which may be or any suitable form and contained in a casing provided with an air blast or other cooling system is connected to-the coutator segments b means of leads which pass longitudina ly through a drum 3" uponwhich the said commutator is mounted, together with certain slip rings, the purpose of which will be described The brush brackets 12 and 13 reepeo neonate tively carried upon radial arms 14 and 15, the hubs-of which are mounted so that their axes are concentric with that of the commutator 3 The arm 14 is fixed to a shaft 16,130 which also is fixed a gear wheel 17, whilst the arm 15 is loosely mounted upon the shaft 16 and is connected by a sleeve 18 engaging by means of a feather 20 or like device a member 19 of an electromagnetic, hand-operated or other clutch 21, the counterpart 22 of which is rigidly attached to a gear wheel 23 having a difierent number of teeth from that of the gear wheel 17, for example, one tooth less. A single pinion 24 engages both the gear wheel 17 and the gear wheel 23 and continuously rotates them by ms of an auxiliary motor 25.

The arm 1a is provided with a projecting tail or stop 26, which is adapted to engage a projection 27 upon the arm 15. A spring 28 is connected between the arms 14 and 15 in such a manner that the projection 27 normally engagesthe stop 26, whereby the brushes 4 and 5 are maintained diametrically opposite one'another, that is to say, in the position which will provide maximum resistance. Rotation of the motor 25 in the clockwise direction (as seen from. the right) causes the wheels 17 and 23 to rotate in the anti-clockwise direction, the latter Wheel idly, so that the arm 14 continuousl rotates in the anti-clockwise direction an carries with it the arm 15 in the same direction by reason of the engagement of the members 26 and 27.

Engagement of the members 19 and 22 of the clutch 21 by electromagnetic, mechanical or other means serves to connect the arm 15 with the continuously rotating wheel 23 having the smaller number of teeth, this wheel being consequently rotated at a greater speed than the wheel 17 so that the arm 15 rotates at a slightly greater speed than the arm 14 whilst both these arms continue to rotate about the commutator 3. This action continuin the brush 5 approaches the brush t until t e resistance is cut out and in order to prevent continued rotation of the arms 14 and 15 when this has occurred, control arrangements as hereinafter described with reference to Fig. 5 may be advantageously provided. Said control arrangements comprise contact members (indicated by dotted lines in F i 3) at 29 and 30 provided on the brush brac ets'12 and 13, respectively, the same contacts engaging one another when the brush arms 1% and 15have approached one another to the full extent. The said members 29' and 30 are insulated from the brushes 4t and 5 and are respectively connected to brushes 31 and 32 which engage slip rings 33 and 34, mounted upon the commutator drum 3 and connected to leads passing tlnrough the end of said drum. The brushes d and 5 electrically cormecmd, respeo tively, with brushes 35 and 36, which engage slip rings 37 and 38, also mounted upon the mutator structure, the resistance being in the self, being either directly driven by saidform of a. plurality of blades 39, which act after the manner of a radial fan, the whole being enclosed in a casing 40, which is shown partly broken away. The said casing may have an axial air inlet 41 and-an outlet 42. The commutator brushes 4 and 5, which are also accommodated within the casing 40, are carried on arms 14 and 15, which may be of exactly similar construction, to the brush arms shown in Fig. 3.

-A pinion 43 is attached to the arm 14 by means of a sleeve rotatably mounted on the shaft 44 of the resistance and commutator structure whilst loosely mounted upon tlie said sleeve is a second wheel 45 with a different number of teeth to which wheel is attached the other brush arm 15. A small pinion 46 is loosely mounted upon a spindle 47, fixed to the member 19 of the magnetic or other clutch 21. When the clutch members are separated, the small pinion 46 engages with the pinion 45, attached to the arm 14, but when the clutch members are brought together the said pinion 46 engages with both of the pinions 43 and 45, and, by rea son of the clutch member 22 being fixed to the rotating shaft 44, one of the arms 14 or 15 rotates about the said shaft towards the other of said arms.

To prevent the arms 14, 15 from being both rotated on account of friction of the brushes on the commutator, either o'f them may be held from movement by means of an external link (not shown) connected, for example, to a lug on the side of the casing 40. As previously stated, the clutch 21 may be of the electromagnetically or the mechanically-operated type, as desired, and further description thereof isconsidered unnecessary since such clutches are well known and the actual construction thereof does not form any part of the present invention.

In some cases, the blade structure 39 may form the cooling fan of the main motor itmain motor or separately driven. In such case, this structure acts as a starting resistance in the first place and subsequently as the ventilating means.

A motor control scheme employingthe rheostat in accordance with the present invention will now be described by way of example.

Referringto Fig. 5, 48 is the armature of a motor to be controlled, for example, a traction motor and 49 is the field winding thereof. It is to be understood that other types of dynamo-electric machines or translating devices may be employed with the rheostat. The said motor is adapted to be connected through the brushes'4 and 5 and resistance 1 to a source of supply such as a trolley 50 and rail or earth 51 by means of a line switch 52 under the control of the driver. The resistance structure may be either of the kind illustrated in Fig. 3, that is to say, with the commutator fixed and the brushesrotating thereon or the kind illustrated in Fig. 4, wherein the resistance and commutator rotate and the brushes are fixed, or both rotations may occur simultaneously. When the brushes 4 and 5 have rotated relative to one another to their full extent, the contacts 29 and 30 engage one another and effect the closure of a short-circuiting switch 53, the operation of which directly connects the brushes 4 and 5 together and stops the rotation of said brushes relative to the commutator. The short-circuiting switch 53 is normally open and is provided with a closing coil 54, which is adaptedto be connected to the supply conductors, when the said auxiliary contacts 29 and 30 engage one another.

The starting up and acceleration of the motor 48 through full resistance and the gradually reduced resistance is initiated by the closure of a control switch 55 or by means of a drum controller, which completes the circuit from the trolley 50 through an actuating coil 56 for the line switch 52, thereby closing the latter. Upon the closure of the line switch 52, the motor 48 is connected to the source of supply through conductor 57, brush 4, the parallel branches of the resistance 1, brush 5, conductor 58, motor armature 48 and field-magnet winding 49 and ground or rail 51.

Another circuit is established upon the closure of the switch 55 through conductor 59, an interlock switch 53+out, which is closed when the short-circuit switch 53 is open and thence to earth through the auxiliary motor, 25, which provides the relative movement between brushes and commutator and between the brushes themselves, to ground. Another circuit. is completed through conductors 59, 61, 62, the actuating coil 63 of a clutch 21 of the electromagnetic type, through an interlock switch 53"out,

which is interlocked with the short-circuitto earth, whilst the conductor 30 is connected on the one hand to trolley through the closing coil 54 of the short-circuiting switch 53 and conductor 59 and on the other hand to earth through an interlock switch 66,

which closes and opens with the short-circuit switch 53.

When the said contacts 29 and 30 eventually engage one another, a circuit is completed from the conductor 59, conductor 61, through the coil 54, contacts 30 and 29, conductor 65 to earth, the interlock switch 53- in being open prior to this moment. The closure of the short-circuiting switch 53 conmeets the brushes 4 and 5 together and also causes interlock switches 53 out to open and interlock switch 53in to close.

The opening of the one interlock switch 53out disconnects the motor 25, whilst the opening of the other interlock switch 53 deenergizes the coil 63 of the clutch 21-so that I the brushes 4 and 5 are separated to the full extent by means of the spring 28, (shown in Figs. 3 and 4). However, as above stated, the closure of the shortcircuiting switch 53 effects the closure of the interlock switch 53- in, which directly connects the coil 54 to the supply and provides a holding circuit for the coil 54 of the switch 53. These con- ,ditions are, therefore, maintaineduntil the control switch 55 is opened. The operation of the auxiliary motor 25, if desired, may be controlled in such manner as to be rotated at a speed proportional to the current traversing the main motor, as will be well understood by those skilled in the art.

Fig. 6 shows a mechanical arrangement for holding the brushes in the short-circuited position when all the resistance has been 'cut out, whereby simplification in the electric circuits may be efi'ected. The fixed brush 4 is provided with a pivoted armature 67 having a hooked shaped tail 68 adapted to en age and hold a counterpart hook member 0 secured to the other brush 5 when the latter approaches the brush 4. The armature 67 is normally urged by means of a compression spring 69 into such a position that the tail 68 will not be engaged by the hook 70. I

An electromagnetic device 71, which is adapted to be connec to the source of supply when the line switch 52 is closed, retains the armature depressed against the action of the spring 69 so that the hook 68 is in a position to be engaged by the hook 70 on the brush 5 when the latter moves into the short-circuiting position adjacent the brush 4. The hooks 68 and 70 remain in engagement until the line switch 52 is opened, whereupon the electromagnet 71 releases the armature 67 and the spring 69 causes the hook 68 to release the hook 70 and permit the brush 5 to be moved back by means of the return spring 28 (see Fig. 3) to its normal position for full resistance diametrically opposite the brush 4. The line switch 52 may be controlled in a similar manner to the corresponding switch in the system illustrated in Fig. 5.

The actuating coil 63 of the magnetic .21 may be operated in other ways than those described and the mechanism for causing the brushes to approach one another may take other forms and the entire operation may be completely non-automatic. In some cases, means may be provided for slowly bringing the brushes together for progressively adding resistance. More than two brushes may play upon the one commutator, for instance for a three-phase starting rheostat, four brushes may be used, two being connected to one of the three phase leads.

' llt will be understood that wide variations in the speed of relative rotation of the brushes and commutator may be made, it being merely necessary that the distribution of the heat amongst the various elements of the rheostat is uniform. The speed of the auxiliary motor in some cases may, there- 'made dependentupon the line current, for

instance by making the field excitation of the auxiliary. motor dependent upon such line current.

Modifications inthe system and arrangement and location of parts may be made without departing from the spirit and spope ofmy invention, and such modifications are intended to be covered by the appended claims. 7

I claim as my invention 1. A rheostat comprising a resistance element arranged in a loop; and a plurality of relatively movable contact members for shunting said element, and means for continuously moving said contact members over said element to effect uniform heating of said element.

2. A rheostat comprising a plurality of resistance elements so arranged as to form a loop, said elements being provided with a plurality of taps, a commutator connected to said taps, a plurality of brushes for enrotatably mounted resistance elements pro vided with openings therebetween for the ventilation thereof.

4:. In a rheostat, the combination with a resistance element having a plurality of taps and a commutator connected to said taps, of a plurality of relatively movable brushes for engaging said commutator, and means for automatically actuating said brushes to vary the amount of said element employed.

5. In an electric motor control system, the combination with a translating device, and a rheostat comprising a commutator and a plurality of brushes for engaging said commutator, and means for automatically shortcircuiting said brushes under predetermined conditions.

6. In an electric motor control system, the

combination with a motor and a rheostat 20 comprising a commutator and a plurality ofbrushes for engaging said commutator, means for automaticay actuating said brushes one toward another, and means for maintaining said brushes in fixed relation NORMAN E. NORTH. 

